The proposed work is focused on the hormonal regulation of testicular function in the laboratory rat. The proposed studies are designed to characterize the amplitude and frequency changes in luteinizing hormone and testosterone secretion under circumstances designed to inhibit or maintain spermatogenesis in the rat. Experiments will be performed to assess the effects of dynamic changes in LH on testosterone production by the in vitro perfused rat testis, investigate the uptake and retention of 5alpha-reduced androgens in the hypothalamus and pituitary gland and to examine the effects of C18 and C19 steroids on germ cell differentiation in long-term hypophysectomized rats.